A sealing technique in a high temperature region over 800° C. will be explained below by taking examples.
First, pure oxygen production and oxygen-rich air production will be described. In this technique, low-cost oxygen or oxygen-rich air is supplied, which brings about an enormous economic effect in industries such as steel, glass, and cement where a large amount of oxygen is consumed. The principle of producing pure oxygen or oxygen-rich air using a mixed-conducting oxide which has oxide ion conductivity and electronic conductivity at the same time is based on the phenomenon that, when two kinds of gases having different oxygen partial pressures are segregated by the mixed-conducting oxide, oxygen in a form of oxide ions permeates the oxide from the high oxygen partial pressure side to the low oxygen partial pressure side.
For example, an oxygen-containing mixed gas (such as air) is compressed to have higher oxygen partial pressure than a gas to be collected (pure oxygen or oxygen-rich air) so that an oxygen gas is separated from the oxygen-containing mixed gas. Efficiency in separating the oxygen gas depends on thickness of the mixed-conducting oxide, difference in oxygen partial pressure on the both sides, and oxide ion conductivity, among which the finally listed conductivity greatly changes depending on temperature, and therefore a temperature region of 800° C. or higher is selected practically. If a gas sealing property is poor in this temperature region, such problems arise that purity of obtained oxygen lowers or production efficiency of oxygen-rich air lowers.
As a second example, a membrane reactor represented by that for partial oxidation of a hydrocarbon gas in which the mixed-conducting oxide is also used will be described. The technology of making a natural gas into liquid fuel (gas to liquid=GTL) has become focused from the viewpoint of the effective use of natural resources, and a technique to be described here is important for the technology as an elemental technique thereof. The principle of the membrane reactor is that an oxygen-containing gas (air, for example) and a hydrocarbon gas (a natural gas mainly composed of methane, for example) are segregated by the mixed-conducting oxide so that oxygen permeates the oxide from the air side to the hydrocarbon gas side, and the hydrocarbon gas is oxidized on a surface of the oxide on the hydrocarbon gas side thereby obtaining a synthesized gas (a mixed gas of carbon monoxide and hydrogen) or a partially oxidized body. Similarly to the oxygen production described above, 800° C. or higher temperature is selected as operation temperature. A poor gas sealing property in this temperature region not only becomes a big factor of reduction in reaction efficiency but also causes complete combustion of hydrocarbon without stopping in extreme cases, which produces a risk of explosion.
As a third example, a solid oxide fuel cell using an oxide ion conducting oxide will be described whose power generation efficiency is high and which is focused as a clean power generation method of an environmentally friendly type. This technology has an advantage that, since a fuel cell is operated at high temperature, total energy efficiency of 70 to 80% can be finally expected if waste heat is used for cogeneration, and research and development are currently conducted intensively. The operation principle of the solid oxide fuel cell is that a fuel gas such as hydrogen and air are segregated by the oxide ion conducting oxide and oxide ions move through the oxide so that electric power is obtained. Yttria stabilized zirconia (YSZ) which is currently being developed is known for having high ion conductivity among oxide ion conducting oxides, but its conductivity is lower than that of the aforesaid mixed-conducting oxide. Therefore, an operation temperature region of a solid oxide fuel cell using YSZ is 900° C. or higher. Also in this technology, a poor gas sealing property may possibly become the prime cause of reduction in output or cause the worst cases such as explosion.
As stated above, a sealing technique in the high temperature region over 800° C. has very important meanings and various sealing methods have been thought, many of which can be seen in a fuel cell region where the development is most advanced. In a case of a fuel cell having a flat-plate structure, a part between a battery cell and a separator (or an interconnector) needs to be sealed. As a sealing material, a ceramic adhesive, various glasses such as borosilicate glass or sodium silicate glass, a heat-resistant metal gasket, a sintered body obtained by firing oxide fine powders, and the like are known.
Japanese Patent Application Laid-open No. Hei 5-325999 describes a sealing material composed of an oxide of a binary or higher system whose solid phase forms a matrix and whose liquid phase functions as a sealing material in a solid-liquid coexisting range of not lower than solidus temperature and not higher than liquidus temperature by controlling a composition ratio of sodium silicate glass.
Japanese Patent Application Laid-open No. Hei 6-231784 describes a sealing material in which a metal foil reinforced by ceramic fiber is used as an aggregate and sodium silicate glass is held by the aggregate.
Japanese Patent Application Laid-open No. Hei 8-7904 describes that a separator is previously heat-treated in the oxygen atmosphere to form an oxide layer on a surface thereof so that compatibility between the separator and a grass sealing material is increased, thereby improving a sealing property.
Japanese Patent Application Laid-open No. Hei 9-115530 describes a method in which a concave part and a protruding part are provided in an upper and a lower surface of a separator respectively so as to make a tenon-joint structure in which these parts are fit, and a heat-resistant metal gasket is inserted between the separator and a solid electrolyte to have their surfaces come in contact with each other so that air tightness is secured.
Japanese Patent Application Laid-open No. Hei 10-116624, Japanese Patent Application Laid-open No. Hei 10-12252, and Japanese Patent Application Laid-open No. Hei 11-154525 describe a solid electrolyte fuel cell using, as a sealing material, a sintered body of raw material powders which are mainly composed of an ultra-fine particle oxide having higher melting point than operation temperature of the solid electrolyte fuel cell.
Japanese Patent Application Laid-open No. Hei 9-129251 describes a sealing method in which a material containing ingredients of both of two materials to be bonded is used as a sealing material in a solid electrolyte fuel cell.
While the techniques described above are directed to the flat-plate type fuel cell, a part between a cylindrical cell and a partition plate for holding it needs to be sealed in a case of a fuel cell having a cylindrical structure.
Japanese Patent Application Laid-open No. Hei 5-29010 and Japanese Patent Application Laid-open No. Hei 5-29011 describe a solid electrolyte fuel cell in which glass is used for sealing between a cylindrical cell and a flange and between the flange and a gas sealing plate (partition plate).
Further, as for a sealing technique in other regions than the fuel cell, P. S. Maiya and et al. (U.S. Pat. No. 5,725,218), the entire disclosure of which is incorporated herein by reference, describes disclose a sealing technique for a part between an Inconel and a solid electrolyte (SFC-2) in a membrane reactor which partially oxidizes methane. Mixed powders of SrO, B2O3, and SrFeCu0.5Ox oxides are selected as a sealing material and they are heated and melted so that a sealing property is obtained.
As described above, a gas sealing technique in a temperature region over 800° C. brings about an enormous economic effect as well as it is an elemental technique indispensable for development of the leading-edge technology which solves environmental problems.
However, the conventional arts are still susceptible to improvement in term of reliability and a heat cycle property although substantial efforts have been made to form a seal, and establishment of a sealing technique in which the seal can be easily formed and which is excellent in reliability and the heat cycle property is desperately desired.
One of the causes of the difficulty in the sealing techniques is thermal expansivity inherent in a material. Specifically, since a temperature region to be used is very high, difference in thermal expansivity between bonding materials becomes more prominent as the temperature increases even though the difference is not so large.
In the present case, linear thermal expansion coefficients of representative materials will be listed below.
Perovskite oxide ion mixed-conducting oxides generally have very high linear thermal expansion coefficients. For example, average linear thermal expansion coefficients from room temperature to 800° C. of La—Sr—Co—Fe mixed-conducting oxides, which are known for high oxide ion conductivity, are approximately 26×10−6/° C. in a case of (La0.2Sr0.8)(Co0.8Fe0.2)Ox and approximately 20×10−6/° C. in a case of (La0.2Sr0.8)(Co0.4Fe0.4CU0.2) Ox. In comparison, linear thermal expansion coefficients of metal are as small as 17.5×10−6/° C. (average of 0 to 650° C.) in a case of stainless steel SUS310S and approximately 14.2×10/° C. (average of 0 to 100° C.) in a case of Incoloy (Incoloy800), and further smaller is that of YSZ, which is approximately 10×10/° C. (average of 0 to 1000° C.). Glass shows extremely small linear thermal expansion of around 1×10−6/° C. (average of 20 to 1000° C.).
The conventional arts in which glass is used as a sealing material utilize the fact that a glass part melts at usage temperature over 800° C. so that a liquid seal having high air tightness can be realized.
However, if the melted glass is used as the sealing material, such problems arise that the sealing material elutes from a bonded portion during usage or the melted glass cannot resist pressure difference when segregated two kinds of gases do not have the same pressure as in the pure oxygen production described above. Further, the glass material also has such problems that high adhesive strength cannot be obtained, a stable property cannot be obtained because alteration of the sealing material such as vaporization or crystallization of ingredients occurs during long-term usage at high temperature, the sealing property cannot be maintained after several heat cycles because of thermal expansion difference if the glass solidifies at low temperature, and chemical reaction with a material to be bonded (particularly, an oxide solid electrolyte) occurs to degrade the material to be bonded.
Japanese Patent Application laid-open No. Hei 10-116624 gazette, Japanese Patent Application laid-open No. Hei 10-12252 gazette, Japanese Patent Application laid-open No. Hei 11-154525 gazette, Japanese Patent Application laid-open No. Hei 9-129251 gazette, and U.S. Pat. No. 5,725,218 described above aim at solving the problem caused by difference in thermal expansivity and giving a stable gas sealing property and a heat-resistant cycle property even in long-term usage at high temperature because thermal expansivity of a sealing material is close to thermal expansivity of two kinds of materials to be bonded.
However, there are some cases where firing temperature of the sealing material is close to firing temperature of the two materials to be bonded, or higher than firing temperature of one of the materials to be bonded depending on combination, which leads to a problem that the material to be bonded is damaged by heat in the step of firing the sealing material. Further, since there are also such problems that the method is troublesome in that the sealing material is formulated and sintered as required, and a sealing property is still susceptible to improvement, the techniques have not been practically used yet.
The present invention is made considering the problems described above, and it is an object of the present invention to provide a composite body in which a seal can be easily formed and a sealing property excellent in reliability and heat cycle property is realized in a high temperature region of 800° C. or higher, a manufacturing method thereof, and a device using the composite body.
Further, the present invention intends to realize a highly efficient applied device for oxygen ion transportation by an idea associated with a usage pattern of oxygen ion transporting ceramic. Specifically, it is an object of the present invention to assemble the highly efficient applied device for oxygen ion transportation by providing an optimal composite structure for transporting oxygen ions to integrate and fix a tubular body, in which a porous ceramic base is covered with an oxygen ion transporting ceramic dense film, to a reaction container.
It is another object of the present invention to provide a ceramic-metal composite structure in which a gas sealing property at high temperature is given to a bonded part between a bonded body obtained by bonding at least one kind of ceramic selected from oxide ion conducting ceramic, electronic conducting ceramic, and mixed-conducting ceramic, or a plurality of the ceramic, and a metal member, and which is excellent in durability against repetition of high temperature and room temperature and maintainability, and a preferred method of producing the same.
It is still another object of the present invention to provide a gasket which gives a gas sealing property at high temperature to the bonded part between the ceramic and the metal member and which is excellent in durability against repetition of high temperature and room temperature and maintainability.
It is yet another object of the present invention to provide a low-cost applied device for oxygen transportation of high reliability and high efficiency in which the ceramic-metal composite structure is included in a case and a gas sealing property at high temperature is given to the bonded part between the ceramic and the metal member, and which has excellent durability against repetition of high temperature and room temperature and improves maintainability.